Process for producing crystalline sodium silicates having a layered structure

ABSTRACT

For producing crystalline sodium silicates having a layer structure, an SiO 2  /Na 2  O molar ratio of (1.9 to 2.1):1 and a water content of less than 0.3% by weight from a waterglass solution containing at least 20% by weight of solids, the waterglass solution is obtained by reacting quartz sand with sodium hydroxide solution at an SiO 2  /Na 2  O molar ratio of (2.0 to 2.3):1 at temperatures of 180° to 240° C. and pressures of 10 to 30 bar. This waterglass solution is treated in a spray-drying zone with hot air at 200° to 300° C. for a residence time of 10 to 25 seconds and at a temperature of the exit gas leaving the spray-drying zone of 90° to 130° C., to form a pulverulent amorphous sodium silicate having a water content (determined as the loss on ignition at 700° C.) of 15 to 23% by weight and a bulk density of more than 300 g/l. The pulverulent, amorphous, water-containing sodium silicate is introduced into an obliquely arranged rotary kiln fitted with devices for moving solids and treated therein with flue gas in countercurrent at temperatures from more than 500° up to 850° C. for 1 to 60 minutes to form crystalline soidium silicate. The rotary kiln is here insulated in such a way that its outside wall temperature is less than 60° C. Finally, the crystalline sodium silicate emerging from the rotary kiln is comminuted by means of a mechanical crusher to grain sizes of 0.1 to 12 mm.

This application is a continuation of application Ser. No. 07/595,433 filed on Oct. 11, 1990, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a process for producing crystalline sodium silicates having a layered structure, an SiO₂ /Na₂ O molar ratio of (1.9 to 2.1):1 and a water content of less than 0.3% by weight from a waterglass solution containing at least 20% by weight of solids.

BACKGROUND AND PRIOR ART

From U.S. Pat. No. 3,471,253, it is known to obtain a waterglass solution by introducing 42% by weight sodium hydroxide solution and sand (silica) in a weight ratio of about 2:1 into a stirred autoclave and allowing the mixture to remain therein for 3 hours at 210° C. and 16 bar. The hot sodium silicate solution taken out after cooling of the autoclave content to 85° C. contains, after excess sand and other impurities have been filtered off, 57.5% of solids and has an SiO₂ /Na₂ O ratio of 1.64:1.

Crystalline, anhydrous sodium silicates having a layer structure and an SiO₂ /Na₂ O molar ratio of (1.9 to 3.5):1 are produced by the process according to German Offenlegungsschrift 3,718,350 by treating waterglass solutions having a solids content of 20 to 65% by weight in a spray-drying zone to form a water-containing amorphous sodium silicate, the exit gas flowing out of the spray-drying zone having a temperature of at least 140° C. The water-containing amorphous sodium silicate is heat-treated in an ignition zone for 1 to 60 minutes at 500° to 800° C. in the presence of at least 10% by weight of recycle material, which was obtained by mechanical comminution of crystalline sodium silicate previously discharged from the ignition zone.

A disadvantage in the last mentioned process is that the material obtained in spray-drying takes up a large volume because of its low bulk density of 100 to 250 g/l and generates a lot of dust. Moreover, the use of recycle material during the heat treatment causes considerably greater expense on equipment and, because of the higher throughput of material, requires a rotary tube of greater dimensions. Finally, due to the use of recycle material at an SiO₂ /Na₂ O molar ratio of 2:1, a high proportion of the high-temperature modification of sodium disilicate (α--Na₂ Si₂ --O₅) is formed but it is not the high-temperature modification which is desired, but the ⊕-modification because of its better builder properties.

SUMMARY OF THE INVENTION

According to the invention, the said disadvantages in the production of crystalline sodium silicates having a layer structure from a waterglass solution containing at least 20% by weight of solids are overcome by

a) obtaining the waterglass solution by reacting quartz sand with sodium hydroxide solution in an SiO₂ /Na₂ O molar ratio of (2.0 to 2.3):1 at temperatures from 180° to 240° C. and pressures from 10 to 30 bar,

b) treating the waterglass solution in a spray-drying zone with hot air at 200° to 300° C. for a residence time of 10 to 25 seconds and at a temperature of the exit gas leaving the spray-drying zone of 90° to 130° C., to form a pulverulent amorphous sodium silicate having a water content (determined as the loss on ignition at 700° C.) of 15 to 23% by weight and a bulk density of more than 300 g/l,

c) introducing the pulverulent, amorphous, water-containing sodium silicate into an obliquely arranged rotary kiln fitted with devices for moving solids and treating it therein with flue gas in counter-current at temperatures from more than 500° up to 850° C. for 1 to 60 minutes to form crystalline sodium silicate, the rotary kiln being insulated in such a way that its outside wall temperature is less than 60° C., and

d) comminuting the crystalline sodium silicate emerging from the rotary kiln by means of a mechanical crusher to grain sizes of 0.1 to 12 mm.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Furthermore, the process according to the invention can, if desired, also be further developed by

aa) grinding the comminuted sodium silicate by means of a mill to grain sizes of 2 to 400 μm;

bb) using a mechanical mill running at a circumferential speed of 0.5 to 60 m/second;

cc) using an air jet mill;

dd) using a ceramically lined ball mill;

ee) using a ceramically lined vibratory mill;

ff) extracting the exit gas from the rotary kiln in the central region thereof and in the region of the end where the pulverulent amorphous sodium silicate is introduced, and purifying the exit gas by means of a dry dust filter, the sodium silicate taken from the dry dust filter being quasi-continuously admixed to the pulverulent, amorphous, water-containing sodium silicate destined to be introduced into the rotary kiln;

gg) feeding the ground crystalline sodium silicate to a roll compactor, by means of which it is compressed at a roll-pressing force of 20 to 40 kN/cm of roll width to give compact pieces;

hh) processing the compact pieces, after pre-comminution by forcing them through screens, to give granules having a bulk density of 700 to 1,000 g/l.

Crystalline sodium silicates are suitable as reinforcing fillers in natural and synthetic rubber. They also behave as ion exchangers and can therefore be used as a sequestrant.

In the process according to the invention, a sodium silicate of high bulk density which can readily be handled, is obtained owing to the low temperature and the short residence time in the spraying of the waterglass solution.

Due to the low heat transfer through the wall of the rotary kiln because of its good insulation, the tendency of the sodium silicate to stick is counteracted in the process according to the invention.

In the process according to the invention, the use of a low-speed mechanical mill (for example a disk mill, beater mill, hammer mill or roll mill) is necessary in order to avoid abrasion of iron from the grinding tools.

If a ceramically lined ball mill or a vibratory mill or an air jet mill for very fine products, i.e. those having diameters of 6 to 10 μm is used in the process according to the invention, likewise no contamination of the sodium silicate due to metal abrasion occurs.

In the process according to the invention, the dust loading in the exit gas is considerably reduced by the simultaneous extraction of dust-containing exit gas in the central region of the rotary tube and in the region of its charging end, because dust is released above all during charging of the sodium silicate to the rotary kiln and because the gas velocity is reduced in the region where the amorphous, water-containing sodium silicate is charged.

Using the process according to the invention, an abrasion-resistant granulated product, which very quickly disintegrates in water, is obtained by compacting.

If a waterglass solution having an SiO₂ /Na₂ O molar ratio of (2.0 to 2.1):1 is used in the process according to the invention, a well crystallized sodium disilicate which is mainly in the form of the δ-modification, has a layer structure, is free of SiO₂ and has a lime-combining capacity of at least 80 mg of Ca/g at 20° C. is obtained in the flue gas treatment in the rotary kiln at temperatures from 600° to 800° C.

EXAMPLE 1 (according to the state of the art)

In a hot-air spray tower (exit gas temperature 145° C.), amorphous sodium disilicate having a water content (determined as the loss on ignition at 700° C.) of 19% and a bulk density of 220 g/l was produced from a waterglass solution having a solids content of 45%.

60 kg/hour of amorphous sodium disilicate and 15 kg/h of a recycle material, which had been obtained by comminution of a product, obtained in a previous batch, to less than 250 μm, were charged via a metering screw to a directly fired rotary kiln (length: 5 m; diameter: 78 cm; inclination: 1.2°) at its end opposite the flame, while the crystalline product was discharged from the flame side. The temperature at the hottest point in the rotary kiln was 740° C.

No material sticking to the wall of the rotary kiln was formed; the discharged crystalline sodium disilicate was largely pulverulent and had a lime-combining capacity of 74 mg of Ca/g.

EXAMPLE 2 (according to the invention)

Sand (99% by weight of SiO₂ ; grain size: 90%<0.5 mm) and 50% by weight sodium hydroxide solution in an SiO₂ /Na₂ O molar ratio of 2.15:1 were filled into a nickel-lined cylindrical autoclave fitted with a stirrer device. With the autoclave being stirred, the mixture was heated to 200° C. by injecting steam (16 bar) and held for 60 minutes at this temperature. The content of the autoclave was then let down through a flash vessel into a tank and, after the addition of 0.3% by weight of perlite as a filter aid, filtered at 90° C. through a disk pressure filter to separate off the insoluble matter. As the filtrate, a clear waterglass solution having an SiO₂ /Na₂ O molar ratio of 2.04:1 was obtained The solids content was adjusted to 50% by dilution with water.

The waterglass solution was sprayed in a hot-air spray tower which was fitted with a disk atomizer and which was heated via a gas-fired combustion chamber and connected to a pneumatically cleaning hose filter for precipitating the product, the combustion chamber having been adjusted in such a way that the hot gas entering at the tower top had a temperature of 260° C. The rate of the waterglass solution to be sprayed was adjusted such that the temperature of the silicate/gas mixture leaving the spray tower was 105° C. The residence time was calculated to be 16 seconds from the volume of the spray tower and the gas throughput through the spray tower. The amorphous sodium disilicate precipitated on the hose filter had, at a low dusting tendency, a bulk density of 480 g/l, an iron content of 0.01% by weight, an SiO₂ /Na₂ O ratio of 2.04:1 and a loss on ignition at 700° C. of 19.4%; its mean particle diameter was 52 μm.

The rotary kiln described in Example 1 had been insulated by several plies of mineral wool and a sheet metal jacket in such a way that, at a temperature of 730° C. in the interior of the rotary kiln, a maximum temperature of 54° C. occurred on its outer skin. 60 kg of the amorphous sodium disilicate were introduced per hour into this rotary kiln, no sticky material being formed. The crystalline sodium disilicate (Na₂ Si₂ O₅ having a layer structure) leaving the rotary kiln and showing a water content of 0.1% by weight (determined as the loss on ignition at 700° C.) was comminuted by means of a mechanical crusher to a grain size of less than 6 mm and, after intermediate cooling, ground on a disk mill (diameter: 30 cm) at 400 min⁻¹ to a mean particle diameter of 110 μm, the iron content of the ground product remaining identical to that of the amorphous sodium disilicate.

The exit gas from the rotary kiln was extracted only in the region where the amorphous sodium disilicate was introduced, and fed to a scrubbing tower. 5 kg of sodium disilicate per hour were discharged with the exit gas.

EXAMPLE 3

(according to the invention)

The product obtained according to Example 2 having a mean particle diameter of 110 μm was further comminuted by means of a fluid-bed opposed jet mill with an integrated mechanical classifier device. Depending on the set speed of rotation of the classifier, an attrition-free sodium disilicate having a mean particle diameter of 2 to 15 μm and a water content of 0.18% by weight was obtained, the layer structure remaining unchanged.

EXAMPLE 4

(according to the invention)

The product obtained according to Example 2 was further comminuted by means of a porcelain-lined ball mill filled with corundum balls. An attrition-free sodium disilicate having a mean particle diameter of 5 to 14 μm, depending on the grinding time, was obtained, the layer structure remaining unchanged.

EXAMPLE 5

(according to the invention)

The product obtained according to Example 2 was processed in a roll compactor having a pressing force of the compacting rolls of 30 kN/cm of roll width with subsequent comminution of the flakes in a screen granulator to give dust-free granules having a mean particle diameter of 750 μm, a bulk density of 820 g/l and a high abrasion resistance.

For the determination of the abrasion resistance, 50 g of granules are treated in a rolling ball mill (length: 10 cm; diameter: 11.5 cm; 8 steel balls of 2 cm diameter) for 5 minutes at a speed of rotation of 100 min⁻¹.

After the abrasion test had been carried out, the mean particle diameter was still 585 μm, which corresponds to a decrease of 22%.

EXAMPLE 6

(according to the invention)

Example 2 was repeated with the modification that the exit gas from the rotary kiln was extracted at two points, namely, apart from the region where the amorphous sodium disilicate was introduced, additionally at a point in the rotary kiln which was at a distance of about 2 m from the said introduction region in the direction of the rotary tube axis. The two exit gas streams were combined and the solids contained therein were precipitated by means of a heat-resistant hose filter. The precipitated solids were re-introduced into the rotary kiln together with the amorphous sodium disilicate, so that no sodium disilicate was lost. As a result, the throughput of the rotary kiln rose to 70 kg/hour, but nevertheless there was no sticky material in the interior of the rotary kiln.

EXAMPLE 7

(comparison example)

Example 2 was repeated with the modification that the hot gas entering at the top of the hot-air spray tower had a temperature of 330° C. The temperature of the silicate/gas mixture leaving the spray tower was 140° C. The sodium disilicate precipitated on the hose filter had a bulk density of 250 g/l, a loss on ignition at 700° C. of 17.9% by weight and a mean particle diameter of 60 μm. This sodium disilicate was very dusty.

EXAMPLE 8

(comparison example)

Example 2 was repeated with the modification that a waterglass solution having an SiO₂ /Na₂ O molar ratio of 2.15:1 was prepared and sprayed in the hot-air spray tower to give an amorphous sodium disilicate having an SiO₂ /Na₂ O ratio of 2.15:1. In the rotary kiln at 730° C., this gave a crystalline sodium disilicate which showed, in the x-ray diagram, the lines of the undesired by-product cristobalite (SiO₂) which is responsible for a reduction in the lime-combining capacity and leads to a deterioration in the builder properties.

EXAMPLE 9

(comparison example)

Example 2 was repeated with the modification that the rotary kiln was insulated only in such a way that, at a temperature of 710° C. in the interior of the rotary kiln, a maximum temperature of 205° C. occurred on its outer skin. As a result, large areas of sticking material formed on the inner wall of the rotary kiln, which frequently had to be knocked off mechanically. From the rotary kiln, a very hard, poorly crystallized product was discharged, some of which had the size of footballs and was very difficult to comminute by the mechanical crusher.

EXAMPLE 10

(comparison example)

Example 2 was repeated with the modification that the sodium disilicate comminuted by means of the mechanical crusher was ground to a mean particle diameter of 98 μm, using an impact disk mill at 10,000 min⁻¹. The ground product had a gray tinge and showed an iron content of 0.025% by weight.

EXAMPLE 11

(comparison example)

Example 5 was repeated with the modification that the pressing force of the compacting rolls was only 15 kN/cm of roll width. The resulting granules had a mean particle diameter of 680 μm and a bulk density of 790 g/l. After the abrasion test had been carried out, the mean particle diameter was then only 265 μm, which correspond to a decrease of 61%. The granules were soft, and some of them already disintegrated into smaller agglomerates on packing.

The lime-combining capacity, indicated in the table which follows, of the sodium silicates, obtained in the examples, having a layer structure was determined by the following procedure:

The CaCl₂ solution (corresponding to 300 mg of CaO) was added to 1 1 of distilled water, whereby a water with 30° German hardness was obtained. 1 g of the crystalline sodium silicate obtained in the examples and 0 to 6 ml of a one-molar glycine solution (obtained from 75.1 g of glycine and 58.4 g of NaCl, dissolved with water up to 1 1) were added to 1 1 of the above water, the temperature of which had been adjusted either to 20 or to 60° C., whereupon a pH of 10.4 established itself. The suspension was stirred for 30 minutes at the selected temperature (20° or 60° C.), during which the pH remained stable. Finally, the solution was filtered and the calcium remaining in solution in the filtrate was determined by complexometry. The lime-combining capacity was determined by forming the difference with the original contents.

                  TABLE                                                            ______________________________________                                         Lime-combining capacity of sodium                                              silicate at pH 10.4 (in mg Ca/g Na.sub.2 Si.sub.2 O.sub.5)                     Example          at 20° C.                                                                        at 60° C.                                     ______________________________________                                         1                74       123                                                  2, 3, 4, 5, 10, 11                                                                              82       132                                                  6                84       136                                                  8                75       125                                                  9                77       126                                                  ______________________________________                                     

We claim:
 1. A process for producing crystalline sodium silicates having a layered structure, an SiO₂ /Na₂ O molar ratio of (1.9 to 2.1):1 and a water content of less than 0.3% by weight from a waterglass solution containing at least 20% by weight of solids, which comprisesa) obtaining the waterglass solution by reacting quartz sand with sodium hydroxide solution in an SiO₂ /Na₂ O molar ratio of (2.0 to 2.3):1 at temperatures from 180° to 240° C. and pressures from 10 to 30 bar, b) treating the waterglass solution in a spray-drying zone with hot air at 200° to 300° C. for a residence time of 10 to 25 seconds and at a temperature of the exit gas leaving the spray-drying zone of 90° to 130° C., to form a pulverulent amorphous sodium silicate having a water content (determined as the loss of ignition at 700° C.) of 15 to 23% by weight and a bulk density of more than 300 g/l, c) introducing the pulverulent, amorphous, water-containing sodium silicate into an obliquely arranged rotary kiln fitted with devices for moving solids and being insulated in such a way that its outside wall temperature is less than 60° C., and treating said water-containing sodium silicate in said rotary kiln with flue gas in counter-current at temperatures from more than 500° to 850° C. for 1 to 60 minutes to form crystalline sodium silicate, d) exhausting the exit gas from the rotary kiln in the central region thereof and in the region of the end where the pulverulent amorphous sodium silicate having a water content of 15 to 25% by weight is introduced, purifying said exit gas by means of a dry dust filter, and admixing the sodium silicate taken from the dry dust filter to the pulverulent, amorphous, water-containing sodium silicate destined to be introduced into the rotary kiln with the resultant increase of the throughput of sodium silicate through the rotary kiln per unit of time, e) comminuting the crystalline sodium silicate emerging from the rotary kiln by means of a mechanical crusher to grain sizes of 0.1 to 12 mm, and f) grinding the comminuted crystalline sodium silicate by means of a mill to grain sizes of 2 to 400 μm with the resultant formation of crystalline sodium silicates having a lime-combining capacity of more than 80 mg Ca/g Na₂ Si₂ O₅ at 20° C.
 2. The process as claimed in claim 1, wherein a mechanical mill running at a circumferential speed of 0.5 to 60 m/s is used.
 3. The process as claimed in claim 1, wherein an air jet mill is used.
 4. The process as claimed in claim 1, wherein a ceramically lined ball mill is used.
 5. The process as claimed in claim 1, wherein a ceramically lined vibratory mill is used.
 6. The process as claimed in claim 1, wherein the ground anhydrous sodium silicate is fed to a roll compactor, by means of which it is compressed at a roll-pressing force of 20 to 40 kN/cm of roll width to give compact pieces.
 7. The process as claimed in claim 6, wherein the compact pieces are, after pre-comminution by forcing them through screens, processed to give granules having a bulk density of 700 to 1,000 g/l. 